Aluminum alloy



Patent ed Apr. 24, 1934 UNITED "STATES ALUMINUM ALLOY Ludwig J. Weber,New Kensington, Pa., assignor to Aluminum Company of America,Pittsburgh, Pa.,' a corporation of Pennsylvania No Drawing.

Application September 21, 1932,

, Serial No. 6 4,149 ionim, (01. 75-1) This invention relates toaluminum base alloys containing substantial amounts of magnesium andparticularly to certain modifications of such alloys which have improvedproperties for specific purposes hereinafter to be described.

The'aluminum base alloys containing magnesium to which thisspecification and the appended claim refer are those containing to percent by weight of magnesium, and no zinc, or if zinc is present, then inamounts not exceeding about-1 per cent by weight. It is a characteristicof these alloys that where the alloy is under a high stress, eitherexternal or internal in source, and is exposed to a corrosive condition,as for instance corrosive solutions, gases, etc., a peculiar cracking orstructural failure may occur. This phenomenon is herein designated asstress cracking. The stress cracking of aluminum base alloys containing5'to 15 per cent by weight of magnesium does not at all times occurunder the conditions above noted and, in fact, its occurrence under mildcorrosion conditions and usual conditions of stress is rare. Yet suchstress cracking may occur and does more frequently occur where thestresses and the corrosion condition are severe. Because the type offailure known as stress cracking is more drastic in its detrimentalaction on the structure of the metal than are the. ordinary types offailure induced by corrosion, the elimination of diminution'of this typeof failure is of great importance and it is to this end that the presentinvention is directed.

I have discovered that the addition of a small amount of copper to theabovementioned alloys will greatly diminish the possibility of stresscracking under service conditions. The effect of my discovery is readilydemonstrated.' Take, for example, a condition of service exceedinglymore severe than ordinary servicea specimen of an aluminum base alloycontaining 5 to 15 per cent by weight ,of magnesium is put under asevere and definite stress by bending it. I The specimen is then placedin an aqueous solution containing about 5 per cent by weight of sodiumchloride and about 0.3 per cent by weight of hydrogen peroxide. This isone of the severest corrosion tests known. Under these conditions stress50 cracking occurs, on the average, at the end of 3 to 5 hours. If thissame specimen contains, in accordance with my invention, a small amountof copper, the stress cracking does not appear, on the average and underthe same conditions, .until 70 to 100 hours have elapsed, I

The amount of copper which has this remarkable efiect is 0.05 to 0.20per cent by weight of thetotal alloy. No. further improvement isattained by the addition 'of copper in-amounts much greater than 0.20per cent and, as I have found, further additions are detrimental in thatthe alloy becomes more susceptible to ordinary or pitting types ofcorrosion. Since one of the outstanding properties of aluminum basealloys containing 5 to 15 per cent of magnesium is the resistance of thealloy to pitting or ordinary corrosion, it is apparent that the additionof an element which causes this pitting is detrimental and, therefore,the copper content should not exceed more than 0.20 .per cent by weightof the total alloy.

As above indicated, the presence of zinc-in the alloy is undesirable inamounts greater than 1.0 per cent by weight and, in fact, I prefer thatthe alloy be substantially zinc-free. For some reason not known to me,the presence of zinc in alloys of the composition herein describedplaces the alloy in such condition that the addition of the small amountof copper does not diminish the possibility of stress cracking. In theappended clairh the term substantially free from zinc defines an alloywhich is zinc-free or contains zinc in amount not greater than 1 percent by weight of the total alloy.

Although any good commercial grade of aluminum may be used successfullyin making the alloys, I have found that the alloys are least prone tostress cracking when the aluminum used in their fabrication is ofrelatively high purity and contains 99.6 per cent or more of aluminum. Iprefer, consequently,.to use metal of this purity.

The alloys may be manufactured by any convenient method, the most commonof which comprises melting the aluminum and adding the alloying elementsthereto in the desired proportions.

The alloys may be heated, aged or otherwise thermally treated inaccordance with principles well known to the art to effect changes'intensile and other properties.- The addition to the alloys of 0.05 to0.20 per cent of copper does not. impair the tensile properties of thealloy and may even increase them as may be evidenced by the fact .that'a heat-treated and artificially aged alu- Having thus described myinvention, I claim:'

A-metallic alloy substantially free from zinc and containing 5 to 15 percent by weight of magnesium and. 0.05 to 0.20 per cent by weight of.

copper the balance being substantially aluminum,

the aluminum used in compounding the alloy being at least 99.6 per centpure aluminum.

LU'DWIG .J. WEBER.

